Group III-nitride and its ternary and quaternary compounds are prime candidates for fabrication of visible and ultraviolet high-power and high-performance optoelectronic devices and electronic devices. These devices are typically grown epitaxially as thin films by growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), or hydride vapor phase epitaxy (HVPE). The selection of substrates is critical for determining the III-Nitride growth direction. Some of the most widely used substrates for nitride growth include SiC, Al2O3, and LiAlO2. III-N and its alloys are most stable in the hexagonal würtzite crystal structure, in which the crystal is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. Due to the relative ease of growing planar Ga-face c-planes, virtually all GaN-based devices are grown parallel to the polar c-axis. A negative consequence of this growth direction is that each layer material will suffer from segregation of electrons and holes to opposite faces of the layers due to the spontaneous polarization of the crystal. Furthermore, strain at the interfaces between adjacent layers gives rise to piezoelectric polarization, causing further charge separation within quantum heterostructures. Such polarization effects decrease the likelihood that electrons and holes will interact, a necessity for the operation of light-emitting optoelectronic devices.
One possible approach to eliminating the piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent non-polar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Such symmetry-equivalent planes in III-N (such as a-plane, m-plane and r-plane) are collectively called as non-polar planes. In addition to these non-polar planes, semi-polar plane III-nitride layers and substrates are also possible. Growth on such non-polar and semi-polar planes substrates could yield significantly enhanced device performance compared to equivalent devices grown on c-plane GaN. Thus, the resulting electronic devices, such as high electron mobility transistors; or optoelectronic devices, such as visible and ultraviolet laser diodes and light-emitting diodes may be more efficient and less power consuming.
However, when non-polar III-Nitride layers are deposited on sapphire substrates, threading dislocations, as well as basal stacking faults, are generated due to lattice mismatch. These extended defects observed in the gallium nitride layers are predominantly threading dislocations and basal stacking faults (BSFs), which originate from the GaN/sapphire interface. The threading dislocation density and stacking fault does not appear to decrease with conventional metalorganic chemical vapor deposition and hydride vapor phase epitaxy deposition techniques. Typical threading dislocation densities are of the order of 1×109 cm−2 and stacking fault are in the range of 5×105 cm−2. Besides high defect density, nonpolar GaN grown on the sapphire surface also has rougher surfaces.
In conventional MOCVD the precursors (such tri-methyl gallium, ammonia etc.) are supplied continuously to the reactant chamber. This adduct formation during MOCVD also hampers the subsequent epilayer growths by increasing the number of stacking faults and dislocation density. A novel pulsed metal organic chemical vapor deposition (P-MOCVD) or pulsed atomic layer epitaxy (PALE) in which the precursors are supplied with alternative supply of sources alleviates the above mentioned problem. This alternative or pulsing technique not only suppress the adduct formation but also provides a unique opportunity to bend the dislocation propagation, to deposit monolayers of material thereby decreasing the slip (which is often the reason for stacking fault generation). Thus, P-MOCVD makes an attractive technique for substrate and epilayer growth and device fabrication.
Although the P-MOCVD alleviates some of the potential problems plaguing III-N devices (especially over non-polar substrates and materials) and represents an enabling technology for the growth of non-polar III-N devices, the relatively high defect density in the directly-grown non-polar or semi-polar III-N films reduces the efficiency of subsequently grown devices compared to what could be achieved by homoepitaxial growth on a perfect substrate. There is an ever-increasing effort to reduce the dislocation density in GaN films in order to improve device performance.
The primary means of achieving reduced dislocation and stacking fault densities in polar c-plane GaN films is the use of a variety of lateral overgrowth techniques, including lateral epitaxial overgrowth (LEO, ELO, or ELOG), selective area epitaxy, and Pendeo epitaxy. The essence of these processes is to block or discourage dislocations from propagating perpendicular to the film surface by favoring lateral growth over vertical growth. These dislocation-reduction techniques have been extensively developed for c-plane GaN growth by HVPE and MOCVD. These conventional growth techniques require a high temperature to accommodate the lateral growth for complete coalescence. This high temperature then subsequently brings in additional problems such as material decomposition, mask auto-doping etc.
However, a need exists for methods of growing high-quality, low-defect density non-polar and semi-polar III-N films.